Field of the Invention
The present invention generally relates to an automobile steering system and, more particularly, to a fluid-operated, four wheel steering system for the simultaneous steering of front and rear pairs of automotive wheels.
The development of the multi-wheel steering system in which a plurality of wheel pairs are simultaneously steered by a single steering wheel maneuvered by a driver is not new, however, the practical use of it in passenger cars is a recent trend. The multi-wheel steering system currently designed for four wheel passenger cars is available in two varieties, a mechanical linkage system and a fluid-operated system. An example of fluid-operated, four wheel steering system is disclosed in, for example, the Japanese Laid-open Patent Publication No. 58-183351, laid open to public inspection on Oct. 16, 1983, and is reproduced in FIG. 14 of the accompanying drawings for the purpose of discussion of the prior art believed to be pertinent to the present invention.
Referring to FIG. 14, there is shown a layout of the fluid-operated, four wheel steering system in relation to front and rear pairs of wheels generally identified by 1 and 4, respectively. A steering knuckle for each front wheel 1 has a spindle 7a for the support thereon of the respective front wheel 1 and a knuckle arm 7b, a free end of said knuckle arm 7b being articulatingly coupled with a respective tie rod 8a. Similarly, the rear wheels 4 are supported by respective steering knuckles each having a spindle 9a for the support thereon of the respective rear wheel 4 and a knuckle arm 9b articulated at a free end thereof to a respective tie rod 8b.
The illustrated steering system comprises a steering gear assembly 2 comprised of, for example a connecting rod 11, provided at a substantially intermediate portion thereof with a rack gear and having its opposite ends pivotally connected with the tie rods 8a, and a pinion gear constantly meshed with the rack gear and operatively coupled with a steering shaft 2a. The steering shaft 2a has a steering wheel 2b rigidly mounted on one end thereof remote from the steering gear assembly 2.
A front cylinder 3 having a piston rod 3a whose opposite ends protrude outwardly from the front cylinder housing is rigidly secured to the housing for the steering gear assembly 2 with the piston rod 3a extending parallel to the longitudinal axis of the connecting rod 11. This front cylinder 3 is described as having left-hand and right-hand working chambers separated from each other by a piston member rigidly mounted on a substantially intermediated portion of the piston rod 3a.
A rear cylinder or rear wheel steering cylinder 5 is similar in structure to the front cylinder 3 and has a piston rod 5a whose opposite ends protrude outwardly from the rear cylinder 5 and are in turn pivotally coupled with the respective tie rods 8b. This rear cylinder 5 is also described as having left-hand and right-hand working chambers separated from each other by a piston member rigidly mounted on a substantially intermediate portion of the piston rod 3a. As a matter of practice, the housing for the rear cylinder 5 is rigidly secured to a portion of the automobile body structure with the piston rod 5a extending parallel to the widthwise direction of the automobile body structure.
As shown, a pair of pipe lines, generally identified by 6, are used, one connecting between the left-hand working chamber of the front cylinder 3 and the right-hand working chamber of the rear cylinder 5 and the other connecting between the right-hand working chamber of the front cylinder 3 and the left-hand working chamber of the rear cylinder 5.
For driving the piston rod 3a of the front cylinder 3, a motion translator generally identified by 35 is employed which comprises a gear box, secured rigidly to the housing of the steering gear assembly 2 and having a pinion gear meshed with the rack gear on the connecting rod 11, a cranking lever 35a having one end coupled with the pinion gear in the gear box and the other end pivotally connected with one of the opposite ends of the piston rod 3a through a connecting link 35b. This motion translator 35 is so designed and so structured that the rotation of the steering wheel 2b in one of the opposite directions from a neutral position about the steering shaft 2a can result in an axial sliding motion of the piston rod 3a. More specifically, so far as the steering wheel 2b is rotated in one of the opposite directions through a relatively small angle from the neutral position, the piston rod 3a can be axially moved in one direction, however, the further rotation of the steering wheel 2b through the increased angle can result in reversal of the direction of sliding motion of the piston rod 3a. It is described that the reversal of the direction of motion of the piston rod 3a takes place when the cranking lever 35a being pivoted moves past a position at which it aligns with the connecting link 35b.
Thus, according to the prior art fluid-operated, four wheel steering system, when the steering wheel 2b is rotated a relatively small angle about the steering shaft 2a from the neutral position to turn the front wheels 1 in one direction, the rear wheels 4 can also be turned in the same direction as the front wheels 1, but when the steering wheel 2b is further rotated to turn the front wheels 1 further in said one direction, the rear wheels 4 can be swung in the other direction towards a normal straight position, in which the rear wheels 4 assume a parallel relationship with the longitudinal sense of the automobile body, or a position in which the rear wheels 4 are turned in a direction opposite to the direction in which the front wheels 1 are turned.
The above mentioned Japanese publication describes that the amount of turn of the rear wheel pair in one direction from the normal straight position which the rear wheel pair assumes during the straight run of the vehicle can be differentiated from that of the front wheel pair by selecting either the length of the cranking lever or the ratio of the inner bore diameter of one of the front and rear cylinders relative to that of the other of the front and rear cylinders. This is because, as clearly stated in the above mentioned Japanese publication, the distance over which the piston rod in the rear cylinder determinative of the amount of turn of the rear wheel pair moves corresponds to the amount of a fluid medium forced to flow into the rear cylinder.
The prior art steering system discussed above is satisfactory in that the use of the fluid circuit including the pipe lines provides a freedom of layout of the steering system with a view to a more efficient space utilization within the vehicle envelope, as compared with the mechanical variant wherein a motion transmitting shaft and differential gears are used. However, since the front and rear cylinders are fluid-connected together in a closed fluid circuit, the ratio of turn of the rear wheel pair relative to that of the front wheel pair when the steering wheel is turned in either direction from the neutral position, or the rear-to-front steering ratio for short, tends to be fixed at all times once the system has been designed. This is in contrast to the established notion that the rear-to-front steering ratio if variable according to the vehicle speed would bring about a favorable steerability.